1. Field of the Invention
This invention relates, in general, to electrical control circuits and, more specifically, to electrical control circuits for D.C. powered solenoids.
2. Description of the Prior Art
Spring assisted, electro-magnetically released brakes have long been used in brake assemblies for electric hoists. Such brake assemblies typically include a biased brake lever which operates a brake shoe and urges a disk into or out of engagement with a fixed brake drum to control braking and movement of a load such as an electrical hoist. An electrically actuated solenoid is commonly employed to move the spring and cause pivotal movement of the brake lever.
Solenoids typically are formed with a coil or winding which drives an actuator and it has long been recongnized that such coils require a high amount of current during start-up relative to the normal holding current. As such solenoids have been used in braking circuits, "forcing" circuits have been required to provide the sufficiently high "in rush" current to the solenoid coil required to provide more rapid actuation of the solenoid. After the solenoid is actuated, a much smaller amount of "holding" current is required to keep the solenoid in its "holding" position. By reducing the electrical power provided to the solenoid during the "holding" period, less heat dissipation capability is required of the solenoid and less energy must be dissipated during de-energization which allows for a more rapid release of the solenoid. To meet these requirements, the forcing circuits include many elements. However, the deletion of such circuits leads to a larger overall size for a solenoid, and requires higher forcing and holding currents, thereby resulting in a slower release of the solenoid.
Thus, it would be desirable to provide a D.C. solenoid circuit which overcomes the problems encountered with previously devised D.C solenoid control circuits. It would also be desirable to provide a D.C. solenoid control circuit which is designed to provide rapid energization and thus actuation of the solenoid as well as a fast release of the solenoid upon de-energization. Finally, it would be desirable to provide a D.C. solenoid control circuit which has a simplified construction resulting in a small size.